Optical character of thermally metamorphosed coals of northern England

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Abstract
SUMMARY: The Carboniferous sediments of northern England have formed the context for an optical study of the effects of differing geothermal history on the included organic material. In the north of the area, the Northumbrian Trough contains 5000 m of cyclothemic Carboniferous sediments with frequent coals and is probably underlain by Old Red Sandstone rocks. To the south, the Alston Block comprises 1500 m of basically similar Carboniferous sediments underlain by highly metamorphosed Silurian slates and a Caledonoid granite. The contrasting nature of the basements has produced variable geothermal gradients and consequently varying coal rank throughout the Carboniferous succession.Alteration of the coals and dispersed organic matter in the sediments by the intrusion of a quartz dolerite sill, the Whin Sill, during Stephanian times has been closely examined. The optical changes induced by the thermal metamorphism depend on the initial rank of the sediments. It has been possible to deduce rank profiles through the succession at the time of intrusion; these can be closely related to the character of the basement. The area was subsequently subjected to hydrothermal (lead, zinc) mineralization.

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